When you're listening to a really good song and can't decide what instrument to pretend to play, so you switch up every few seconds. Most common instruments include: lead guitar, rhythm guitar, drums, and keyboard.

I can't listen to Spraynard cuz I always start air banding really hard.

"I can't decide between picking the guitar, strumming the guitar, or drumming, so I'm just gonna air band."
"Dude, who CAN decide?"